ath: move the channel for ath_reg_apply_beaconing_flags() into helper

While at it convert this into a switch statement, this
makes it easier and manage.

Cc: smihir@qti.qualcomm.com
Cc: tushnimb@qca.qualcomm.com
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>